Discrete molecular sieve and use

ABSTRACT

This invention presents a novel MgAPSO molecular sieve, containing a critical range of magnesium in the sieve framework, which is particularly active for hydrocarbon conversion. The sieve advantageously is incorporated, along with a platinum-group metal, into a catalyst formulation which is useful for isomerization. When utilized in a process for isomerizing a non-equilibrium mixture of xylenes containing ethylbenzene, a greater yield of para-xylene is obtained compared to prior-art processes.

CROSS REFERENCE TO RELATED APPLICATION

This application is a division of prior copending application Ser. No.814,749, filed Dec. 26, 1991, the contents of which are incorporatedherein by reference thereto.

FIELD OF THE INVENTION

This invention relates to an improved molecular sieve and its use forthe conversion of hydrocarbons. More specifically, the inventionconcerns a magnesium-containing non-zeolitic molecular sieve which has anarrowly defined composition and is particularly useful forisomerization.

GENERAL BACKGROUND AND RELATED ART

A large variety of molecular sieves have been disclosed in the art asuseful in catalysts for hydrocarbon conversion. The most well known arethe crystalline aluminosilicate zeolites formed from corner-sharing AlO₂and SiO₂ tetrahedra. The zeolites generally feature pore openings ofuniform dimensions, significant ion-exchange capacity and the capabilityof reversibly desorbing an adsorbed phase which is dispersed throughoutthe internal voids of the crystal without displacing any atoms whichmake up the permanent crystal structure. Zeolites often arecharacterized by a critical, usually minimum, silica/alumina ratio.

More recently, a class of useful non-zeolitic molecular sievescontaining framework tetrahedral units (TO₂) of aluminum (AlO₂),phosphorus (PO₂) and at least one additional element EL (ELO₂) has beendisclosed. "Non-zeolitic molecular sieves" include the "ELAPSO"molecular sieves as disclosed in U.S. Pat. No. 4,793,984 (Lok et al.),"SAPO" molecular sieves of U.S. Pat. No. 4,440,871 (Lok et al.) andcrystalline metal aluminophosphates--MeAPOs where "Me" is at least oneof Mg, Mn, Co and Zn--as disclosed in U.S. Pat. No. 4,567,029 (Wilson etal.). Framework As, Be, B, Cr, Fe, Ga, Ge, Li, Ti or V and binary metalaluminophosphates are disclosed in various species patents. Particularlyrelevant to the present invention is U.S. Pat. No. 4,758,419 (Lok etal.), which discloses MgAPSO non-zeolitic molecular sieves. Generallythe above patents teach a wide range of framework metal concentrations,e.g., the mole fraction of (magnesium+silicon) in Lok et al. '419 may bebetween 0.02 and 0.98 with a preferably upper limit of 0.35 molefraction and magnesium concentration of at least 0.01.

The use of catalysts containing a zeolitic molecular sieve and magnesiumfor isomerization is disclosed in U.S. Pat. Nos. 4,482,773 (Chu et al.)and 4,861,740 (Sachtler et al.), but neither of these referencesdisclose an isomerization catalyst containing non-zeolitic molecularsieves. The use of a catalyst containing a MgAPSO non-zeolitic molecularsieve in hydrocarbon conversion including isomerization is disclosed inthe aforementioned U.S. Pat. No. 4,758,419 (Lok et al.). U.S. Pat. No.4,740,650 (Pellet et al.) teaches xylene isomerization using a catalystcontaining at least one non-zeolitic molecular sieve which may beMgAPSO. Neither Pellet et al. nor Lok et al., however, disclose orsuggest the narrow criticality of the magnesium content of anon-zeolitic molecular sieve which is a feature of the presentinvention.

Catalysts for isomerization of C₈ aromatics ordinarily are classified bythe manner of processing ethylbenzene associated with the xyleneisomers. Ethylbenzene is not easily isomerized to xylenes, but itnormally is converted in the isomerization unit because separation fromthe xylenes by superfractionation or adsorption is very expensive. Awidely used approach is to dealkylate ethylbenzene to form principallybenzene while isomerizing xylenes to a near-equilibrium mixture. Analternative approach is to react the ethylbenzene to form a xylenemixture in the presence of a solid acid catalyst with ahydrogenation-dehydrogenation function. The former approach commonlyresults in higher ethylbenzene conversion, thus lowering the quantity ofrecycle to the para-xylene recovery unit and concomitant processingcosts, but the latter approach enhances xylene yield by forming xylenesfrom ethylbenzene. A catalytic composition and process which enhanceconversion according to the latter approach, i.e., achieve ethylbenzeneisomerization to xylenes with high conversion, would have significantutility.

SUMMARY OF THE INVENTION

A principal object of the present invention is to provide a novelmolecular sieve which is useful for the conversion of hydrocarbons. Morespecifically, this invention is directed to a catalytic compositioncomprising a novel molecular sieve and a process for the isomerizationof a mixture of xylenes and ethylbenzene resulting in improved yieldsand/or reduced processing costs.

This invention is based on the discovery that a MgAPSO molecular sievehaving a framework magnesium content controlled within critical limitsdemonstrates a "volcano" effect in hydrocarbon-conversion activity.

Accordingly, a broad embodiment of the invention is directed toward aMgAPSO molecular sieve having a framework content of magnesium within acritical range. Preferably the sieve is incorporated into a catalyticcomposition comprising a platinum-group metal; the optimal catalyticcomposition also contains an inorganic-oxide matrix. In an alternativeembodiment, the catalytic composition also comprises an AlPO₄ molecularsieve.

Another embodiment is directed toward a process for hydrocarbonconversion using a catalytic composition containing a MgAPSO molecularsieve having a content of magnesium within a critical range. Preferablythe process comprises isomerization, more preferably of a feed streamcomprising a non-equilibrium mixture of xylenes and ethylbenzene atisomerization conditions to obtain a product having an increasedpara-xylene content.

These as well as other objects and embodiments will become evident fromthe following detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 compares the activity k_(A) of MgAPSO molecular sieves having arange of framework magnesium contents.

FIG. 2 relates k_(A) and ethylbenzene conversion for several differentmolecular sieves.

FIG. 3 compares a catalyst of the invention against two catalysts of theprior art with respect to xylene selectivity vs. ethylbenzeneconversion.

FIG. 4 shows the impact of removing ortho-xylene from the feed on theperformance of a catalyst of the invention.

DETAILED DESCRIPTION OF THE INVENTION

As mentioned above, this invention is drawn to a MgAPSO molecular sievehaving a framework content of magnesium within a critical range.

The MgAPSO molecular sieve of the invention can be understood byreference to the disclosure of U.S. Pat. No. 4,758,419, incorporatedherein by reference thereto. MgAPSO sieves have a microporouscrystalline framework structure of MgO₂ ⁻², AlO₂ ⁻, PO₂ ⁺, and SiO₂tetrahedral units having an empirical chemical composition on ananhydrous basis expressed by the formula:

    mR:(Mg.sub.w Al.sub.x P.sub.y Si.sub.z)O.sub.2

wherein "R" represents at least one organic templating agent present inthe intracrystalline pore system; "m" represents the molar amount of "R"present per mole of (Mg_(w) Al_(x) P_(y) Si_(z))O₂ and has a value ofzero to about 0.3; and "w", "x", "y" and "z" represent the molefractions of element magnesium, aluminum, phosphorus and silicon,respectively, present as tetrahedral oxides. The mole fraction of eachframework constituent of the molecular sieve is defined as acompositional value which is plotted in phase diagrams of U.S. Pat. No.4,758,419. The mole fractions "w", "x", "y" and "z" are generallydefined as being within the limiting compositional values or points asfollows:

    ______________________________________                                               Mole Fraction                                                          Point    x            y      (z + w)                                          ______________________________________                                        A        0.60         0.38   0.02                                             B        0.39         0.59   0.02                                             C        0.01         0.60   0.39                                             D        0.01         0.01   0.98                                             E        0.60         0.01   0.39                                             ______________________________________                                    

It is an essential aspect of the present invention that the magnesiumcontent of the MgAPSO sieve is controlled within narrow limits.Specifically, the mol fraction "w" of framework magnesium in themolecular sieves of the invention is between about 0.003 and 0.035. Bestresults are obtained when the mol fraction of framework magnesium isbetween about 0.005 and 0.025.

A "volcano" effect has been observed on butane-cracking activity "k_(A)" when the magnesium content of the sieves is controlled within theabove limits according to the invention. Volcano effect refers to anunusual and surprising increase in k_(A) for sieves of the inventionrelative to sieves having both higher and lower magnesium contents.Butane-cracking activity is a readily determined representation ofhydrocarbon-conversion activity in such processing areas asisomerization, reforming, dehydrocyclization, dehydrogenation,disproportionation, transalkylation, dealkylation, alkylation,polymerization, and catalytic cracking.

The butane cracking activity k_(A) is determined by testing an 0.5 to5-gram sample of 20-40 mesh MgAPSO sieve particles loaded into acylindrical quartz tube, as described more specifically hereinafter inExample II. The quantity of sieves is selected to effect butaneconversion of from 5% to 90% when butane is present in a concentrationof 2 mole % in a helium carrier. The feedstock and reactor effluent areanalyzed by conventional gas chromatography, and the pseudo-first-orderrate constant k_(A) is calculated from the analytical data.

The nomenclature employed herein to refer to the members of the class ofMgAPSOs is consistent with that employed in the aforementioned patents.A particular member of a class is generally referred to as a "-n"species wherein "n" is an integer, e.g., MgAPSO-11, MgAPSO-31 andMgAPSO-41. The especially preferred species of the present invention isMgAPSO-31 having a characteristic X-ray powder diffraction pattern whichcontains at least the d-spacings set forth below:

    ______________________________________                                                                  Relative                                            2θ       d          Intensity                                           ______________________________________                                         8.4-9.501     10.53-9.3084                                                                             w-s                                                 20.2-20.4       4.40-4.35 m                                                   22.0-22.1       4.04-4.022                                                                              m                                                   22.5-22.7       3.952-3.92                                                                              vs                                                  23.15-23.35     2.831-2.814                                                                             w-m                                                 ______________________________________                                    

MgAPSO sieves generally are synthesized by hydrothermal crystallizationfrom an aqueous reaction mixture containing reactive sources ofmagnesium, silicon, aluminum and phosphorus and an organic templatingagent for an effective time at effective conditions of pressure andtemperature. The reaction-mixture compositions preferably are expressedin terms of molar ratios as follows:

    aR:(Mg.sub.r Al.sub.s P.sub.t Si.sub.u)bH.sub.2 O

wherein (r+s+t+u)=1.00 mole such that the aforementioned frameworkconstituents "w", "x", "y" and "z" of the molecular sieves have thecompositional values as described, the amount of organic templatingagent "a" has a preferably positive value between 0 and about 6, and theamount of water "b" is between 0 and 500 with a preferable value between2 and 300.

The organic templating agent, if any, can be selected from among thosedisclosed in U.S. Pat. No. 4,758,419. Generally this agent will containone or more elements selected from Group VA (IUPAC 15) of the PeriodicTable [See Cotton and Wilkinson, Advanced Inorganic Chemistry, JohnWiley & Sons (Fifth Edition, 1988)], preferably nitrogen or phosphorusand especially nitrogen, and at least one alkyl or aryl group havingfrom 1 to 8 carbon atoms. Preferred compounds include the amines and thequaternary phosphonium and quaternary ammonium compounds. Mono-, di- andtri-amines are advantageously utilized, either alone or in combinationwith a quaternary ammonium compound. Especially preferred amines includedi-isopropylamine, di-n-propylamine, triethylamine and ethylbutylamine.

The reaction source of silicon may be silica, either as a silica sol oras fumed silica, a reactive solid amorphous precipitated silica, silicagel, alkoxides of silicon, silicic acid or alkali metal silicate andmixtures thereof.

The most suitable reactive source of phosphorus yet found for theinstant process is phosphoric acid, but organic phosphates such astriethyl phosphate have been found satisfactory, and so also havecrystalline or amorphous aluminophosphates such as the AIPO₄ compositionof U.S. Pat. No. 4,310,440. Organo-phosphorus compounds selected astemplating agents do not, apparently, serve as reactive sources ofphosphorus, but these compounds may be transformed in situ to a reactivesource of phosphorus under suitable process conditions.

The preferred aluminum source is either an aluminum alkoxide, such asaluminum isoproxide, or pseudoboehmite. The crystalline or amorphousaluminophosphates which are a suitable source of phosphorus are, ofcourse, also suitable sources of aluminum. Other sources of aluminumused in zeolite synthesis, such as gibbsite, sodium aluminate andaluminum trichloride, can be employed but are not preferred.

The reactive source of magnesium can be introduced into the reactionsystem in any form which permits the formation in situ of a reactiveform of magnesium, i.e., reactive to form the framework tetrahedral unitMgO₂ ⁻². Compouunds of magnesium which may be employed include oxides,hydroxides, alkoxides, nitrates, sulfates, halides, carboxylates (e.g.acetates and the like), organo-metallics and mixtures thereof.

Crystallization generally is effected in a sealed pressure vessel,preferably lined with an inert plastic material such aspolytetrafluoroethylene. While not essential to the synthesis of MgAPSOcompositions, stirring or other moderate agitation of the reactionmixture and/or seeding the reaction mixture with seed crystals of eitherthe MgAPSO species to be produced or a topologically similaraluminophosphate, aluminosilicate or other molecular sieve compositionfacilitates the crystallization procedure. The reaction mixture ismaintained advantageously under autogenous pressure at a temperaturebetween 50° and 250°, and preferably between 100° and 200° C., for aperiod of several hours to several weeks. The crystallization periodadvantageously will be between about 4 hours and 20 days. The MgAPSOproduct is recovered by any convenient method such as centrifugation orfiltration.

After crystallization the MgAPSO product may be isolated andadvantageously washed with water and dried in air. The as-synthesizedMgAPSO will typically contain within its internal pore system at leastone form of any templating agent, also referred to herein as the"organic moiety", employed in its formation. Most commonly the organicmoiety is present, at least in part, as a charge-balancing cation. Insome cases, the MgAPSO pores are sufficiently large and the organicmolecule sufficiently small that the removal of the latter may beeffected by conventional desorption procedures. Generally, however, theorganic moiety is an occluded molecular species which is too large tomove freely through the pore system of the MgAPSO product and must bethermally degraded and removed by calcining at temperatures of from 200°to 700° C.

The MgAPSO compositions are formed from MgO₂, AlO₂, PO₂ and SiO₂tetrahedral units which, respectively, have a net charge of -2, -1, +1and 0. An AlO₂ ⁻ tetrahedron can be balanced electrically either byassociation with a PO₂ ⁺ tetrahedron or a simple cation such as analkali metal cation, a proton (H⁺), a cation of magnesium present in thereaction mixture, or an organic cation derived from the templatingagent. Similarly, an MgO₂ ⁻² tetrahedron can be balanced electrically byassociation with PO₂ ⁺ tetrahedra, a simple cation such as alkali metalcation, a proton (H⁺), a cation of the magnesium, organic cationsderived from the templating agent, or other divalent or polyvalent metalcations introduced from an extraneous source. Ion exchange of MgAPSOcompositions will ordinarily be possible only after the organic moietypresent as a result of synthesis has been removed from the pore system.

It is within the scope of the invention that a catalytic compositionprepared from the MgAPSO of the invention comprises one or moreadditional non-zeolitic molecular sieves. Preferably the non-zeoliticmolecular sieves are as a multi-compositional, multi-phase compositehaving contiguous phases, a common crystalline framework structure andexhibiting a distinct heterogeneity in composition, especially whereinone phase comprises a deposition substrate upon which another phase isdeposited as an outer layer. Such composites are described in U.S. Pat.No. 4,861,739, incorporated herein by reference thereto. Suitablenon-zeolitic molecular sieves include but are not limited to those ofU.S. Pat. Nos. 4,440,871, 4,567,029 and 4,793,984, incorporated byreference. In a highly preferred embodiment the layered catalyticcomposition comprises a crystalline aluminophosphate of U.S. Pat. Nos.4,310,440, incorporated by reference. The AIPO₄ of this embodiment is acrystalline metallophosphate whose essential framework structure has achemical composition, expressed in terms of molar ratios of oxides, of:

    Al.sub.2 O.sub.3 : 1.0±0.2P.sub.2 O.sub.5

AIPO₄ -31 is especially preferred as a substrate and a MgAPSO,especially MgAPSO-31, as an outer layer.

A catalytic composition preferably is prepared by combining themolecular sieves of the invention with a binder for convenient formationof catalyst particles. The binder should be porous, adsorptive supporthaving a surface area of about 25 to about 500 m² /g, uniform incomposition and relatively refractory to the conditions utilized in thehydrocarbon conversion process. The term "uniform in composition"denotes a support which is unlayered, has no concentration gradients ofthe species inherent to its composition, and is completely homogeneousin composition. Thus, if the support is a mixture of two or morerefractory materials, the relative amounts of these materials will beconstant and uniform throughout the entire support., It is intended toinclude within the scope of the present invention carrier materialswhich have traditionally been utilized in hydrocarbon conversioncatalysts such as: (1) refractory inorganic oxides such as alumina,titanium dioxide, zirconium dioxide, chromium oxide, zinc oxide,magnesia, thoria, boria, silica-alumina, silica-magnesia,chromia-alumina, alumina-boria, silica-zirconia, etc.; (2) ceramics,porcelain, bauxite; (3) silica or silica gel, silicon carbide, clays andsilicates including those synthetically prepared and naturallyoccurring, which may or may not be acid treated, for example attapulgusclay, diatomaceous earth, fuller's earth, kaolin, kieselguhr, etc.; (4)crystalline zeolitic aluminosilicates, either naturally occurring orsynthetically prepared such as FAU, MEL, MFI, MOR, MTW (IUPAC Commissionon Zeolite Nomenclature), in hydrogen form or in a form which has beenexchanged with metal cations, (5) spinels such as MgAl₂ O₄, FeAl₂ O₄,ZnAl₂ O₄, CaAl₂ O₄, and other like compounds having the formula MO-Al₂O₃ where M is a metal having a valence of 2; and (6) combinations ofmaterials from one or more of these groups.

The preferred matrices for use in the present invention are refractoryinorganic oxides, with best results obtained with a binder comprised ofalumina. Suitable aluminas are the crystalline aluminas known as thegamma-, eta-, and theta-aluminas. Excellent results are obtained with amatrix of substantially pure gamma-alumina. In addition, in someembodiments, the alumina matrix may contain minor proportions of otherwell known refractory inorganic oxides such as silica, zirconia,magnesia, etc. Whichever type of matrix is employed, it may be activatedprior to use by one or more treatments including but not limited todrying, calcination, and steaming.

Using techniques commonly known to those skilled in the art, thecatalytic composition of the instant invention may be composited andshaped into any useful form such as spheres, pills, cakes, extrudates,powders, granules, tablets, etc., and utilized in any desired size.These shapes may be prepared utilizing any known forming operationsincluding spray drying, tabletting, spherizing, extrusion, andnodulizing.

A preferred shape for the catalyst composite is an extrudate. Thewell-known extrusion method initially involves mixing of thenon-zeolitic molecular sieve, either before or after adding metalliccomponents, with the binder and a suitable peptizing agent to form ahomogeneous dough or thick paste having the correct moisture content toallow for the formation of extrudates with acceptable integrity towithstand direct calcination. Extrudability is determined from ananalysis of the moisture content of the dough, with a moisture contentin the range of from 30 to 50 wt. % being preferred. The dough then isextruded through a die pierced with multiple holes and thespaghetti-shaped extrudate is cut to form particles in accordance withtechniques well known in the art. A multitude of different extrudateshapes are possible, including, but not limited to, cylinders,cloverleaf, dumbbell and symmetrical and asymmetrical polylobates. It isalso within the scope of this invention that the extrudates may befurther shaped to any desired form, such as spheres, by any means knownto the art.

An alternative shape of the composite is a sphere, continuouslymanufactured by the well-known oil drop method. Preferably, this methodinvolves dropping the mixture of molecular sieve, alumina sol, andgelling agent into an oil bath maintained at elevated temperatures. Thedroplets of the mixture remain in the oil bath until they set and formhydrogel spheres. The spheres are then continuously withdrawn from theoil bath and typically subjected to specific aging treatments in oil andan ammoniacal solution to further improve their physicalcharacteristics. The resulting aged and gelled particles are then washedand dried at a relatively low temperature of about 50°-200° C. andsubjected to a calcination procedure at a temperature of about 450°-700°C. for a period of about 1 to about 20 hours. This treatment effectsconversion of the hydrogel to the corresponding alumina matrix.

A preferred component of the present catalytic composition is aplatinum-group metal including one or more of platinum, palladium,rhodium, ruthenium, osmium, and iridium. The preferred platinum-groupmetal is platinum. The platinum-group metal component may exist withinthe final catalyst composite as a compound such as an oxide, sulfide,halide, oxysulfide, etc., or as an elemental metal or in combinationwith one or more other ingredients of the catalytic composition. It isbelieved that the best results are obtained when substantially all theplatinum-group metal component exists in a reduced state. Theplatinum-group metal component generally comprises from about 0.01 toabout 2 mass % of the final catalytic composite, calculated on anelemental basis.

The platinum-group metal component may be incorporated into the catalystcomposite in any suitable manner. The preferred method of preparing thecatalyst normally involves the utilization of a water-soluble,decomposable compound of a platinum-group metal to impregnate thecalcined zeolite/binder composite. For example, the platinum-group metalcomponent may be added to the calcined hydrogel by commingling thecalcined composite with an aqueous solution of chloroplatinic orchloropalladic acid.

It is within the scope of the present invention that the catalyticcomposition may contain other metal components known to modify theeffect of the platinum-group metal component. Such metal modifiers mayinclude rhenium, tin, germanium, lead, cobalt, nickel, indium, gallium,zinc, uranium, dysprosium, thallium, and mixtures thereof. Catalyticallyeffective amounts of such metal modifiers may be incorporated into thecatalyst by any means known in the art.

The catalytic composition of the present invention may contain a halogencomponent. The halogen component may be either fluorine, chlorine,bromine or iodine or mixtures thereof. Chlorine is the preferred halogencomponent. The halogen component is generally present in a combinedstate with the inorganic-oxide support. The halogen component ispreferably well dispersed throughout the catalyst and may comprise frommore than 0.2 to about 15 wt %, calculated on an elemental basis, of thefinal catalyst.

The halogen component may be incorporated in the catalytic compositionin any suitable manner, either during the preparation of theinorganic-oxide support or before, while or after other catalyticcomponents are incorporated. For example, the carrier material maycontain halogen and thus contribute at least some portion of the halogencontent in the final catalyst. The halogen component or a portionthereof also may be added to the catalyst during the incorporation ofother catalyst components into the support, for example, by usingchloroplatinic acid in impregnating a platinum component. Also, thehalogen component or a portion thereof may be added to the catalyst bycontacting with the halogen or a compound, solution, suspension ordispersion containing the halogen before or after other catalystcomponents are incorporated into the support.

The catalyst composite is dried at a temperature of from about 100° toabout 320° C. for a period of from about 2 to about 24 or more hours andcalcined at a temperature of from 400° to about 650° C. in an airatmosphere for a period of from about 0.1 to about 10 hours until themetallic compounds present are converted substantially to the oxideform. The optional halogen component may be adjusted by including ahalogen or halogen-containing compound in the air atmosphere.

The resultant calcined composite may be subjected to a substantiallywater-free reduction step to insure a uniform and finely divideddispersion of the optional metallic components. Preferably,substantially pure and dry hydrogen (i.e., less than 20 vol. ppm H₂ O)is used as the reducing agent in this step. The reducing agent contactsthe catalyst at conditions, including a temperature of from about 200°to about 650° C. and for a period of from about 0.5 to about 10 hours,effective to reduce substantially all of the Group VIII metal componentto the metallic state.

The resulting reduced catalytic composite may, in some cases, bebeneficially subjected to a presulfiding operation designed toincorporate in the catalytic composite from about 0.05 to about 0.5 mass% sulfur calculated on an elemental basis. Preferably, this presulfidingtreatment takes place in the presence of hydrogen and a suitablesulfur-containing compound such as hydrogen sulfide, lower molecularweight mercaptans, organic sulfides. etc. Typically, this procedurecomprises treating the reduced catalyst with a sulfiding gas such as amixture of hydrogen and hydrogen sulfide having about 10 moles ofhydrogen per mole of hydrogen sulfide at conditions sufficient to effectthe desired incorporation of sulfur, generally including a temperatureranging from about 10° up to about 593° C. or more. It is generally agood practice to perform this presulfiding step operation undersubstantially water-free conditions.

MgAPSO sieves of the invention are useful for the conversion ofhydrocarbons to obtain a converted product. The sieves preferably areutilized in combination with at least one inorganic-oxide matrix and oneor more metals as described herein. A hydrocarbon feedstock is convertedat hydrocarbon-conversion conditions including a pressure of aboutatmospheric to 200 atmospheres, temperatures of about 50° to 600° C.,liquid hourly space velocities of from about 0.1 to 100 hr⁻¹, and ifhydrogen is present, hydrogen-to-hydrocarbon molar ratios of from about0.1 to 80.

Hydrocarbon-conversion processes which would advantageously employcatalytic compositions containing the MgAPSO sieves of the inventioninclude isomerization, reforming, dehydrocyclization, dehydrogenation,disproportionation, transalkylation, dealkylation, alkylation,polymerization, hydrocracking and catalytic cracking.

MgAPSO catalyst compositions used in reforming processes preferablycontain a hydrogenation promoter such as a platinum-group metal,optionally one or more modifiers such as rhenium and Group IVA (14)metals, and an inorganic-oxide binder. Hydrocarbon feedstocks,preferably naptha, contact the catalyst at pressures of betweenatmospheric and 40 atmospheres, temperatures of about 350° to 600° C.,liquid hourly space velocities (LHSV) from 0.2 to 20 hr⁻¹, andhydrogen-to-hydrocarbon molar ratios of from about 0.1 to 10.Dehydrocyclization of naphthas and other paraffin-containing stocks iscarried out over a similar catalyst, preferably nonacidic throughincorporation of an alkali or alkaline earth metal, at similarconditions with operating pressure no higher than about 15 atmospheres.Products of reforming and dehydrocyclization generally have an increasedconcentration of aromatics relative to the feedstocks.

Isomerization of light hydrocarbons is advantageously effected usingMgAPSO catalyst compositions within the scope of those described for usein reforming processes. The light hydrocarbon feedstock contacts thecatalyst at pressures of between atmospheric and 70 atmospheres,temperatures of about 50° to 300°, LHSV from 0.2 to 5 hr⁻¹, andhydrogen-to-hydrocarbon molar ratios of from about 0.1 to 5.Isomerization of olefins such as butenes, pentenes and higher olefins iseffected over a catalyst which preferably does not contain a substantialhydrogenation component, in order to avoid olefin hydrogenation, atsomewhat higher temperatures of 200° to 600° C. and higher spacevelocities of 0.5 to 100 hr⁻¹. Usually isomerization yields a producthaving a greater concentration of branched hydrocarbons.

Heavier paraffins, waxy distillates and raffinates are isomerized toincrease the branching of the hydrocarbons using essentially the samecatalyst compositions as used in reforming. Operating conditions includepressures of between about 20 and 150 atmospheres, temperatures of about200° to 450° C., LHSV from 0.2 to 10 hr⁻¹, and hydrogen-to-hydrocarbonmolar ratios of from about 0.5 to 10.

MgAPSO catalyst compositions used in hydrocracking processes preferablycontain a hydrogenation promoter such as one or more of Group VIII(8-10) and Group VIB (6) metals and an inorganic-oxide matrix. A varietyof feedstocks including atmospheric and vacuum distillates, cycle stocksand residues are cracked to yield lighter products at pressures ofbetween 30 and 200 atmospheres, temperatures of about 200° to 450° C.,LHSV from 0.1 to 10 hr⁻¹, and hydrogen-to-hydrocarbon molar ratios offrom about 2 to 80.

Catalyst compositions of the same general description as those used inhydrocracking processes are useful in hydrotreating and hydrofining. Avariety of naphthas, atmospheric and vacuum distillates, cracked andcycle stocks and residues are treated to remove sulfur, nitrogen andother heteroatoms and to saturate unsaturates at pressures of between 30and 150 atmospheres, temperatures of about 200° to 450° C., LHSV from0.1 to 20 hr⁻¹, and hydrogen-to-hydrocarbon molar ratios of from about 2to 20. Operating conditions vary with respect to the difficulty ofheteroatom removal, usually relating to the size and aromaticity of thecontaining molecules, and the concentration particularly of nitrogen inthe feedstock. Products meet environmental requirements, are not ascorrosive or contaminating of downstream equipment, or effect lessdeactivation of catalysts in downstream-processing units relative to thefeedstock.

Disproportionation also is effected with MgAPSO catalyst compositions asdescribed above in relation to reforming processes; optionally, thecatalyst also contains one or more Group VIA (6) metals. Suitablefeedstocks include single-ring aromatics, naphthalenes and lightolefins, and the reaction yields more valuable products of the samehydrocarbon specie. Isomerization and transalkylation also may occur atthe operating conditions of between 10 and 70 atmospheres, temperaturesof about 200° to 500° C., and LHSV from 0.1 to 10 hr⁻¹. Hydrogen isoptionally present at a molar ratio to hydrocarbon of from about 0.1 to10.

A particularly advantageous use for the MgAPSO sieves of the inventionis in the isomerization of isomerizable alkylaromatic hydrocarbons ofthe general formula C₆ H.sub.(6-n) R_(n), where n is an integer from 2to 5 and R is CH₃, C₂ H₅, C₃ H₇, or C₄ H₉, in any combination andincluding all the isomers thereof to obtain more valuable isomers of thealkylaromatic. Suitable alkylaromatic hydrocarbons include, for example,ortho-xylene, meta-xylene, para-xylene, ethylbenzene, ethyltoluenes,trimethylbenzenes, diethylbenzenes, triethyl-benzenes,methylpropylbenzenes, ethylpropylbenzenes, diisopropylbenzenes, andmixtures thereof.

Isomerization of a C₈ -aromatic mixture containing ethylbenzene andxylenes is a particularly preferred application of the MgAPSO sieves ofthe invention. Generally such mixture will have an ethylbenzene contentin the approximate range of 5 to 50 mass %, an ortho-xylene content inthe approximate range of 0 to 35 mass %, a meta-xylene content in theapproximate range of 20 to 95 mass % and a para-xylene content in theapproximate range of 0 to 15 mass %. It is preferred that theaforementioned C₈ aromatics comprise a non-equilibrium mixture, i.e., atleast one C₈ -aromatic isomer is present in a concentration that differssubstantially from the equilibrium concentration at isomerizationconditions. Usually the non-equilibrium mixture is prepared by removalof para-and/or ortho-xylene from a fresh C₈ aromatic mixture obtainedfrom an aromatics-production process.

The alkylaromatic hydrocarbons may be utilized in the present inventionas found in appropriate fractions from various refinery petroleumstreams, e.g., as individual components or as certain boiling-rangefractions obtained by the selective fractionation and distillation ofcatalytically cracked or reformed hydrocarbons. The isomerizablearomatic hydrocarbons need not be concentrated, but may be present inminor quantities in various streams. The process of this inventionallows the isomerization of alkylaromatic-containing streams such ascatalytic reformate with or without subsequent aromatics extraction toproduce specified xylene isomers, particularly para-xylene. A C₈-aromatics feed to the present process may contain nonaromatichydrocarbons i.e., naphthenes and paraffins, in an amount up to 30 mass%.

According to the process of the present invention, an alkylaromatichydrocarbon charge stock, preferably in admixture with hydrogen, iscontacted with a catalyst of the type hereinabove described in analkylaromatic hydrocarbon isomerization zone. Contacting may be effectedusing the catalyst in a fixed-bed system, a moving-bed system, afluidized-bed system, or in a batch-type operation. In view of thedanger of attrition loss of the valuable catalyst and of the simpleroperation, it is preferred to use a fixed-bed system. In this system, ahydrogen-rich gas and the charge stock are preheated by suitable heatingmeans to the desired reaction temperature and then passed into anisomerization zone containing a fixed bed of catalyst. The conversionzone may be one or more separate reactors with suitable meanstherebetween to ensure that the desired isomerization temperature ismaintained at the entrance to each zone. The reactants may be contactedwith the catalyst bed in either upward-, downward-, or radial-flowfashion, and the reactants may be in the liquid phase, a mixedliquid-vapor phase, or a vapor phase when contacted with the catalyst.

The alkylaromatic charge stock, preferably a non-equilibrium mixture ofC₈ aromatics, is contacted with a catalytic combination as hereinbeforedescribed in an isomerization zone while maintaining the zone atappropriate alkylaromatic-isomerization conditions. The conditionscomprise a temperature ranging from about 0° to 600° C. or more, andpreferably is in the range of from about 300° to 500° C. The pressuregenerally is from about 1 to 100 atmospheres absolute, preferably lessthan about 50 atmospheres. Sufficient catalyst is contained in theisomerization zone to provide a liquid hourly space velocity of chargestock of from about 0.1 to 30 hr⁻¹, and preferably 0.5 to 10 hr⁻¹. Thehydrocarbon charge stock optimally is reacted in admixture with hydrogenat a hydrogen/hydrocarbon mole ratio of about 0.5:1 to about 25:1 ormore. Other inert diluents such as nitrogen, argon and lighthydrocarbons may be present.

The particular scheme employed to recover an isomerized product from theeffluent of the reactors of the isomerization zone is not deemed to becritical to the instant invention, and any effective recovery schemeknown in the art may be used. Typically, the reactor effluent will becondensed and the hydrogen and light-hydrocarbon components removedtherefrom by flash separation. The condensed liquid product then isfractionated to remove light and/or heavy byproducts and obtain theisomerized product. In some instances, certain product species such asortho-xylene may be recovered from the isomerized product by selectivefractionation. The product from isomerization of C₈ aromatics usually isprocessed to selectively recover the para-xylene isomer, optionally bycrystallization. Selective adsorption is preferred using crystallinealuminosilicates according to U.S. Pat. No. 3,201,491. Improvements andalternatives within the preferred adsorption recovery process aredescribed in U.S. Pat. Nos. 3,626,020, 3,696,107, 4,039,599, 4,184,943,4,381,419 and 4,402,832, incorporated herein by reference thereto.

In a separation/isomerization process combination relating to theprocessing of an ethylbenzene/xylene mixture, a fresh C₈ -aromatic feedis combined with isomerized product comprising C₈ aromatics andnaphthenes from the isomerization reaction zone and fed to a para-xyleneseparation zone; the para-xylene-depleted stream comprising anon-equilibrium mixture of C₈ aromatics is fed to the isomerizationreaction zone, where the C₈ -aromatic isomers are isomerized tonear-equilibrium levels to obtain the isomerized product. In thisprocess scheme non-recovered C₈ -aromatic isomers preferably arerecycled to extinction until they are either converted to para-xylene orlost due to side-reactions. Ortho-xylene separation, preferably byfractionation, also may be effected on the fresh C₈ -aromatic feed orisomerized product, or both in combination, prior to para-xyleneseparation.

The following examples are presented for purpose of illustration onlyand are not intended to limit the scope of the present invention.

EXAMPLES

The following examples are presented for purpose of illustration onlyand are not intended to limit the scope of the present invention. Theexamples demonstrate the criticality of magnesium content in molecularsieves of the invention by butane-cracking activity, relatebutane-cracking and isomerization activity, and demonstrate the utilityof the catalyst for isomerization of C₈ aromatics.

MgAPSO-31 compositions have been prepared and tested employing reactionmixtures having a molar composition expressed as:

    aR:rMgO:sAl.sub.2 O.sub.3 :tP.sub.2 O.sub.5 :uSiO.sub.2 :bH.sub.2 O:

wherein the values a, r, s, t, u and b represent moles of template R,magnesium (expressed as the oxide), Al₂ O₃, P₂ O₅ (H₃ PO₄ expressed asP₂ O₅), SiO₂ and H₂ O, respectively. The values ranged as follows, basedon t=1:

    ______________________________________                                        a              1.5-2.0                                                        r              as described hereinbelow                                       s              0.75-1.1                                                       u              0.1-1.2 (usually 0.6)                                          b              40-80                                                          ______________________________________                                    

These ranges do not, however, limit the applicability of the presentinvention as described hereinabove.

EXAMPLE I

Tests are reported below for MgAPSO-31 compositions prepared viareaction mixtures having a molar composition of about:

    1.5R:rMgO:0.9Al.sub.2 O.sub.3 :P.sub.2 O.sub.5 :0.3SiO.sub.2 :50H.sub.2 O:

The value r was varied to provide a range of mol fractions of frameworkmagnesium in the context of the previously defined formula:

    (Mg.sub.w Al.sub.x P.sub.y Si.sub.z)

wherein (w+x+y+z)=1.00 and w is the mol fraction of framework magnesium.

The reaction mixture was prepared by mixing the Al₂ O₃ as pseudoboehmite(Versal 250) into the H₃ PO₄ and water on a gradual basis and blendinguntil a homogeneous mixture was observed. Magnesium acetate wasdissolved in a portion of the water and then was added followed byaddition of LUDOX-LS. The combined mixture was blended until ahomogeneous mixture was observed. The organic templating agent(ethylbutylamine) and AlPO₄ -31 seed were added to this mixture andblended until a homogeneous mixture was observed. Portions of theresulting mixture were placed in either lined (polytetrafluoroethylene)stainless steel pressure vessels for quiescent crystallization or anunlined stirred stainless steel pressure vessel and heated up to about200° C. to effect crystallization at autogenous pressure. The productswere removed from the reaction vessel, cooled and evaluated as set forthhereinafter.

The examples present test results obtained when catalysts of theinvention were evaluated in an isomerization process. The catalysts wereevaluated using a pilot plant flow reactor processing a non-equilibriumC₈ aromatic feed comprising 52.0 mass % meta-xylene, 18.5 mass %ortho-xylene, 0.1 mass % para-xylene, 21.3 mass % ethylbenzene, and 0.1mass % toluene, with the balance being nonaromatic hydrocarbons. Thisfeed was contacted with 100 cc of catalyst at a liquid hourly spacevelocity of 2, and a hydrogen/hydrocarbon mole ratio of 4. Reactorpressure and temperature were adjusted to cover a range of conversionvalues in order to develop the relationship between C₈ ring loss andapproach to xylene equilibrium (as determined by product para-xylene tototal xylene mole ratio). At the same time, at each temperature, thepressure was chosen to maintain a constant mole ratio of C₈ naphthenesto C₈ aromatics of approximately 0.06.

EXAMPLE II

A representation of the hydrocarbon-conversion activity of the presentclass of medium-pore molecular sieves is the butane-cracking activity"k_(A) " determined using a bench-scale apparatus. This activitymeasurement allows larger number of samples to be surveyed with moreconsistent results than, e.g., isomerization performance in a pilotplant. The reactor is a cylindrical quartz tube having a length of 254mm and an I.D. of 10.3 mm. In each test the reactor was loaded with20-40 mesh (U.S. std.) particles of the MgAPSO-31 molecular sieve in anamount of from 0.5 to 5 grams, the quantity being selected so that theconversion of n-butane was at least 5% and not more than 90% under thetest conditions. As-synthesized samples containing organics are firstcalcined in situ in the reactor in air at 600° C. for one hour to removeorganic materials from the pore system, then in a flowing stream ofhelium at 500° C. for at least 10 minutes. The activity k_(A) then wasdetermined using a feedstock consisting of a helium/n-butane mixturecontaining 2 mole percent n-butane which is passed through the reactorat a rate of 50 cc/minute. The feedstock and the reactor effluent wereanalyzed using conventional gas chromatography techniques, reactoreffluent being analyzed after 10 minutes of on-stream operation at 500°C. The pseudo-first-order rate constant k_(A) was calculated from theanalytical data.

Eighteen samples of MgAPSO-31 with varying magnesium contents, and twocontrols without magnesium, were prepared according to the procedure ofExample I. Crystallization was carried out at 200° C. with eight samplesin a quiescent reaction mixture and ten stirred samples. The activityk_(A) of the samples was determined according to the procedure describedhereinabove and plotted in FIG. 1. The samples showed particularly highactivities in the region of 0.005-0.025 mole fraction of magnesium, withsome increased activity at 0.003 and 0.03-0.035 mole fraction magnesiumand low activities at the outer limits of the tests. Activities of theeleven most active samples in the middle of the range were twice orthree times those of samples containing the highest concentrations offramework magnesium.

EXAMPLE III

Butane-cracking activity k_(A) was related to ethylbenzene conversion.Seven non-zeolitic molecular-sieve samples were tested for ethylbenzeneconversion at a pressure of 20 atmospheres, temperature of 427° C., andmass hourly space velocity of 4 using a feed of 17 mass % ethylbenzeneand 83 mass % meta-xylene. Ethylbenzene ("EB") conversion was chosen asthe measure of comparison since ethylbenzene is the most difficult ofthe C₈ -aromatics isomers to convert in an isomerization process. Theactivity k_(A) also was determined for the seven samples in accordancewith the procedure of Example II. Results were as follows, and also areplotted in FIG. 2:

    ______________________________________                                        Catalyst       EB Conversion, %                                                                            k.sub.A                                          ______________________________________                                        SAPO-31        11            0.7                                              MgAPSO-31      34            3.8                                              MnAPSO-31      25            2.4                                              MnAPO-31       28            1.7                                              CoAPO-31       41            2.3                                              CoAPSO-31      29            2.1                                              CoAPSO-31      44            3.7                                              ______________________________________                                    

There is a clear correlation between k_(A) and ethylbenzene conversion,even though there is some scatter in the data points as would beexpected from the testing of a variety of molecular sieves.

EXAMPLE IV

The utility of the catalytic composition of the present invention wasdemonstrated by measuring selectivity to xylenes at varying ethylbenzeneconversions. Xylene selectivity is defined as the ratio ofpotential/actual xylenes in the product.

The catalyst base contained 50 mass % of MgAPSO-31, containing 0.013 molfraction magnesium, and 50 mass % alumina. The finished catalystcontained, in mass %:

    ______________________________________                                               platinum       0.24%                                                          chloride       0.25%                                                          sulfur         0.07%                                                   ______________________________________                                    

Results were compared to those for prior-art catalysts which had beendemonstrated to be effective for isomerization of C₈ aromatics. Oneprior-art catalyst was SAPO-11, amounting to 40 mass % of a base alongwith 40% alumina and 20% silica, in a catalyst containing 0.48 mass %platinum and prepared according to U.S. Pat. No. 4,740,650. Anotherprior-art catalyst comprised gallium-modified ZSM-5 according to U.S.Pat. No. 4,957,891.

The feedstock used in this example had the following composition in mass%:

    ______________________________________                                               ethylbenzene    17%                                                           meta-xylene     58%                                                           orthoxylene     25%                                                    ______________________________________                                    

The results are plotted in FIG. 3 for the catalysts of the invention andof the prior art. The catalyst of the invention shows an advantage ofabout 2% in xylene selectivity over the gallium-modified-ZSM-5 catalystand an even greater advantage over the prior-art SAPO-11 catalyst.

EXAMPLE V

Isomerization of an essentially pure ethylbenzene feed was investigatedusing the catalyst of the invention. The alumina-bound MgAPSO-31catalyst of Example IV was employed at 427° C. and mass space velocitiesvarying from 6 to 9 as required to achieve a range of ethylbenzeneconversions. The proportion of para-xylene in the xylene portion of theproduct was compared to the thermodynamic equilibrium value at 427° C.Results were as follows:

    ______________________________________                                        Ethylbenzene   Para-Xylene                                                    Conversion %   % of Equilibrium                                               ______________________________________                                        21.8           173                                                            24.1           168                                                            29.5           155                                                            ______________________________________                                    

The surprising super-equilibrium yield of para-xylene may indicate,without limiting the invention, that the mechanism of ethylbenzeneconversion is selective to para-xylene production.

EXAMPLE VI

Isomerization performance of the catalyst of the invention wasinvestigated when processing a feedstock from which ortho-xylene hadbeen removed. Ortho-xylene, being an industrially important intermediateas noted hereinabove, often is separated from mixed xylene isomers in anaromatics complex by fractionation. In this embodiment of aseparation/isomerization process combination, fresh C₈ -aromatic feed iscombined with isomerized product comprising C₈ -aromatics and naphthenesfrom the isomerization reaction zone and fed to an fractionator whichseparates orthoxylene product in a bottoms stream. Overhead from theorthoxylene fractionator is sent to the para-xylene separation zone; thepara-xylene depleted stream is fed to the isomerization reaction zone,where the C₈ -aromatic isomers are again isomerized to near-equilibriumlevels. In this process scheme the C₈ -aromatic isomers are recycled toextinction, until they are either converted to ortho- or para-xylene orlost due to side-reactions.

The ortho- and para- depleted feed consisted essentially of 17 mass %ethylbenzene and 83 mass % meta-xylene as described in Example III.Para-xylene content of the xylene portion of the isomerized product wascompared to the thermodynamic-equilibrium value at 427° C. in order todetermine the % of para-xylene equilibrium and whether asuper-equilibrium in excess of 100% had been reached. The results areplotted in FIG. 4: % of para-xylene equilibrium was over 100% for a widerange of ethylbenzene conversions, and approached 120% at 40%ethylbenzene conversion.

We claim:
 1. A hydrocarbon-conversion process which comprises contactinga hydrocarbon feedstock, at hydrocarbon-conversion conditions, with acatalytic composition comprising a crystalline MgAPSO molecular sievewhich contains from about 0.003 to 0.035 mol fraction of magnesium inthe microporous crystalline framework structure to obtain a convertedproduct.
 2. The process of claim 1 wherein the content of magnesium inthe microporous crystalline framework structure is from about 0.005 to0.025 mol fraction.
 3. The process of claim 1 wherein the MgAPSOcomprises MgAPSO-31.
 4. The process of claim 1 wherein the non-zeoliticmolecular sieve further comprises an AIPO₄.
 5. The process of claim 1wherein the catalytic composition further comprises an inorganic-oxidematrix.
 6. A process for the isomerization of a non-equilibrium feedmixture of xylenes and ethylbenzene comprising contacting the feedmixture in the presence of hydrogen in an isomerization zone atalkylaromatic-isomerization conditions, with a catalytic compositioncomprising at least one platinum-group metal component and MgAPSO-31which contains from about 0.003 to 0.035 mol fraction of magnesium inthe microporous crystalline framework structure, to produce anisomerized product.
 7. The process of claim 6 wherein thealkylaromatic-isomerization conditions comprise a temperature of fromabout 300° to 500° C., a pressure of from about 1 to 50 atmospheres, aliquid hourly space velocity of from about 0.5 to 10 hr⁻¹ and ahydrogen-to-hydrocarbon mole ratio of from about 0.5:1 to 25:1.
 8. Theprocess of claim 6 wherein the content of magnesium in the MgAPSO-31 isfrom about 0.005 to 0.025 mol fraction in the microporous crystallineframework structure.
 9. The process of claim 6 wherein theplatinum-group metal component comprises from about 0.1 to 5 mass %platinum on an elemental basis.
 10. The process of claim 1 wherein thecatalyst further comprises an inorganic-oxide matrix.
 11. The process ofclaim 6 further comprising recovery of para-xylene by selectiveadsorption from the isomerized product and a fresh C₈ -aromatic feed.12. The process of claim 11 wherein ortho-xylene is recovered from oneor both of the isomerized product and fresh C₈ -aromatic feed.
 13. Theprocess of claim 12 wherein the isomerized product contains agreater-than-equilibrium concentration of para-xylene.